The "water softeners" that I know of use a synthetic cation exchange resin charged with sodium ions. The *divalent* cations (Ca++, Mg++, Fe++, etc) have a stronger affinity for the resin than the monovalent sodium (Na+) ion and displace the sodium ions. The calcium and other ions stay stuck to the resin until they are displaced during backflushing with very high *concentrations* of sodium ions, and so the calcuim ions are flushed out of the filter as waste with the brine solution, and fresh sodium ions are put back where they started. Postassium chloride (KCl) or hydrochloric acid (HCl) can be used in place of sodium chloride (NaCl).

Phosphate ions come in two flavors, hydrogen phosphate (HPO4--) and dihydrogen phosphate (H2PO4-), and then there is phosphoric acid (H3PO4). The deal here is that all three of these substances are present when you add any one of them, and by varying the concentrations of the dissolved parts, you change the pH. If you add disodium hydrogen phosphate (Na2HPO4) to pure water the pH should go to 10.9. Add potassium dihydrogen phosphate (KH2PO4), the pH should go to 5.8. Add phosphoric acid, the pH goes to below 3. So you can prepare a mix of salts that will buffer your pH to any value you want between 3 and 10. Thatís why itís so commonly used in the pH up/down/buffer products.

Hydrogen phosphate will also chelate calcium and magnesium, which prevents them from reacting with soap, thatís why it used to be added to laundry soap, so that the soap would clean, instead of reacting with the hard water. Perhaps this use was what you heard about relative to softening the water? I havenít heard of phosphate being used in water softeners. Potassium, yes, phosphate, no.

I agree with you on the calcium carbonate (CaCO3) reacting with carbonic acid (H2CO3) to form calcium Ca++) and two bicarbonate ions 2(HCO3-). The water softener removes the calcium and replaces it with two sodium ions. The main thing here is that the alkalinity does not change. The water softener leaves the bicarbonates alone. The disassociation constant for the new pair of ions is different, but sodium bicarbonate is pretty soluable, so that does not explain the drop to 6.5 from 7.8. So, my guess is that the water company is adding a strong base to the water which brings the pH up to 7.8, and additionally the anion part of the base forms an insoluable salt with sodium. Or, when the sodium is added to the water, the additive which was used to drive the pH up reacts with sodium and precipitates out. I'm an engineer, not a chemist, so I could be way off base, but the pH being inconsistant with the alkalinity says that says to me that something is added to the water that the softener is removing in some way. Another possibility is that the carbon prefilter is leaching whatever acid was used during to process the carbon.